The single sideband is generated independently by digital synthesis and transmitted over wire lines or radio frequency channels for detection by a detector circuit that does not require the re-insertion of the carrier. The method has a very narrow bandwidth as transmitted, being much narrower, or higher in terms of bandwidth efficiency in bits/second/Hz, than currently employed methods. The invention has broad utility in radio, microwave, and satellite applications.
Various digital modulation schemes are known to those skilled in the art. These methods have varying bandwidth efficiencies that have been calculated in various texts and analyzed for other characteristics as well. One such summary is given in a paper by this inventor published in Applied Microwave and Wireless Magazine, July/August 1997. In general, most of the existing methods modulate a carrier which is suppressed in the modulator, then recovered and re-inserted in the receiver. Single sideband digital modulation is possible but has been rarely utilized to date. One such method is the Variable Phase Shift Keying (VPSK) method covered in U.S. Pat. Nos. 4,742,532 and 5,185,765 issued to the present inventor. This method utilizes a special data encoding method and single sideband transmission to achieve bandwidth efficiencies of as high as 15.3 Bits/sec./Hz. A carrier is required to recreate the single sideband suppressed carrier signal and to decode the signal in the receiver.
It has been found that proper encoding of the signal at baseband can result in a very narrow frequency band being occupied at baseband and that this can in turn be transmitted SSB-SC in a very narrow RF bandwidth. Bandwidth efficiencies as high as 50 bits/second/Hz have been demonstrated in the laboratory, with 26-27 b/s/Hz being achieved in usable hardware. A comparison of this new method, which has been named Very Minimum Shift Keying (VMSK), with the older VPSK method, appeared in the January 1997 issue of the IEEE Transactions on Broadcasting.
It has been found that the previously used methods for SSB digital modulation, which involved baseband encoding, a carrier and filtering to remove the unwanted sideband, are unnecessarily complex. The characteristics of the sideband alone are reproducible by direct digital synthesis. Although the signal can be detected by normal means, that is by re-constructing a carrier and reinserting it in the detector to recover the encoded baseband signal, this is not necessary. There is sufficient information in the sideband alone to be detected without a carrier.
High bandwidth efficiency methods generally lose power as a result of the bandwidth compression. That is not the case with the present invention, which theoretically maintains the same signal to noise ratio, regardless of bandwidth efficiency. This seemingly violates Carson's Rule and Shannon's Limit. As will be shown, this is not the case with the present invention.